The invention disclosed herein relates generally to capacitive keyboard systems, and more particularly to low cost membrane capacitive key switch systems in which sense node to ground capacitance required for detector operation is provided without the requirement for a discrete conductive ground plane.
Capacitive keyboards are well known in the field of data input devices for information handling and data processing systems. They provide certain inherent advantages over electrical contact keyboards. The advantages particularly include mechanical simplicity, long life and absence of electrical signal problems caused by contact bounce and corrosion. Keyboards using capacitive key switches also facilitate the provision of N-key roll over which permits proper detection of key switch actuations in the presence of multiple switches remaining actuated during the same time period.
In order to reduce the cost and complexity of detection circuitry used with capacitive keyboards, and in order to reduce the number of conductors necessary to connect such keyboard with external detection circuitry, it is known to provide for sequential scanning of key actuated capacitors to determine key status. This may be accomplished by means of a matrix of row and column conductors, each key actuated capacitor being connected between a unique pair of conductors comprising one row and one column conductor. A drive signal may be sequentially impressed on the conductors in one set of conductors while the conductors in the other set are sequentially addressed to detect signals coupled thereto.
There is, however, a problem with scanned capacitive key switch systems which stems from the fact that there is inherent capacitive coupling between every pair of elements in a keyboard. Thus, in a typical capacitive keyboard having a matrix of conductors including drive signal and sense signal conductors, a signal on any conductor will normally appear to some extent on every other conductor. Obviously, this phenomenon complicates the task of detecting which of an array of variable capacitors between pairs of conductors in the matrix is actuated.
Various capacitance detection schemes have been devised for satisfactorily dealing with the above described detection problem. One such detector circuit is described in detail in U.S. Pat. No. 4,728,931 issued to W.J. Linder, et al. on Mar. 1, 1988 and assigned to the assignee of the present application. Satisfactory implementation of this detector circuit, as well as most somewhat similar circuits, depends on careful balancing of the actuated and unactuated key switch capacitances, the sense node to ground capacitance and the capacitance value of a reference capacitor in the detector circuit.
As apparent from the above identified patent, the detector circuit disclosed therein is well suited for fabrication as an integrated circuit using standard CMOS integrated circuit designs which provide for low power consumption and low cost. Such designs are sensitive to input signal voltage parameters. In particular, input signal voltages less than the circuit reference voltage by more than a predetermined amount may cause latch-up and destruction of the circuit. In prior capacitive keyboard designs using metal housings and other components which furnished a conductive backplane, the sense node to ground capacitance provided inherent protection against excessively negative sense node voltages.
More recent keyboard designs directed at high volume keyboard markets which have become increasingly cost competitive have tended away from metal components and toward all plastic construction. This greatly reduces the sense node to ground capacitance and the control it provided over excessively negative input signal voltages. It is apparent that the sense node to ground capacitance could be reintroduced by adding a metallized membrane backplane or equivalent elements. However, the addition of parts is undesirable from manufacturing cost considerations.
The applicant has devised a unique membrane capacitive keyboard design which retains the advantages of prior membrane capacitive keyboards and detector circuit designs while eliminating the need for metal housings or backplanes and without introducing additional parts or manufacturing steps.